Method and device for magnetic detection of binding of biological molecules on a biochip in which a magnetoresistive sensor device measures an areal density of magnetic nanoparticles on a micro-array, the magnetic nanoparticles being directly or indirectly coupled to a target sample. The magnetoresistive sensor device includes a substrate having attached thereto binding sites able to selectively bind the target sample, and a magnetoresistive sensor for detecting the magnetic field of the nanoparticles coupled to the target sample. The magnetoresistive sensor includes a plurality of magnetoresistive sensing elements, the width and length dimensions of which are at least a factor 10 or more, preferably a factor 100 or more larger than the diameter of the nanoparticles.